The present invention relates to a DC-DC converter, and more particularly to the DC-DC converter capable of reducing input current ripples for improving circuit efficiency.
U.S. Pat. No. 5,910,712 discloses a DC-DC converter of the type known as a fly-back converter which includes a transformer with a primary winding connected in series with a switching element across an input DC power source and a secondary winding connected across a smoothing output capacitor that is responsible for supplying an output DC voltage to a load. In operation, the switching element is controlled to turn on and off for repetitively interrupting the input DC voltage supplied to the primary winding so as to accumulate the energy in the primary winding when the switching element is on and release the corresponding energy from the secondary winding to charge the smoothing output capacitor when the switching element is off, thereby providing a smoothed DC output voltage to the load. Thus, it is possible to set the output DC voltage at a desired level even lower than the input DC voltage by selecting a duty cycle of the switching element.
This circuit, however, permits no input current being supplied from the input DC power source while the switching element is off, thereby suffering from increased input current ripples. The increased ripples results in lowering the circuit efficiency as well as corresponding increased input current peak which necessitates a large capacity for the transformer with attendant increase in the bulk of the transformer. Also, since the transformer in this circuit is alone responsible for conveying the energy from the input DC power source to the load, the transformer has to include a relatively large core in order to prevent magnetic flux saturation and is therefore made into a large bulk. Thus, it is difficult to use the transformer of compact design and to assemble the whole circuit into a compact sufficient to be installed within a limited space.
In view of the above insufficiency, the present invention has been accomplished to provide an improved DC-DC converter with reduced input current ripples which is capable of utilizing a compact transformer for reducing the physical dimensions of the converter, as well as to provide a ballast for a discharge lamp making the use of the compact DC-DC converter. The DC-DC converter in accordance with the present invention has a converter input which is adapted to receive an input DC voltage, and a converter output which is adapted to be connected to a load for proving an output DC voltage to the load. The converter includes a transformer having a primary winding and a secondary winding. The primary winding is connected in series with a switching element across the converter input. The switching element is driven to turn on and off in order to repetitively interrupt the DC input voltage and induce an energy at the secondary winding in response to the switching element being turned off. A capacitor is connected in circuit to be charged by the energy released from the secondary winding so as to accumulate the output DC voltage, and is connected across the converter output to provide the resulting output DC voltage to the load.
The characterizing feature of the present invention resides in that the capacitor is connected in series with a rectifier and the secondary winding so as to be charged by the energy released from the secondary winding through the converter input. With this arrangement, the circuit sees an input current which continues flowing through the converter input even while the switching element is turned off. Thus, no interruption in the input DC current is assured to thereby reduce the input current ripples and therefore the input current peak which enables the use of small-sized transformer for overall compact arrangement of the converter, yet improving the circuit efficiency.
A controller is included in the converter to determine a switching frequency of the switching element that is sufficiently higher than a resonance frequency given to a resonant system given by the capacitor and the secondary winding in order to restrain undesirable resonance for reliable converter operation.
In one embodiment of the present invention, the capacitor is connected in series with the primary winding across the DC power source so as to form a closed loop of the capacitor, the secondary winding, the rectifier, the converter input and the primary winding for flowing the input current therethrough while the switching element is off. For this purpose, the secondary winding has a polarity chosen in relation to the winding sense of the primary winding such that the input DC voltage is superimposed in phase upon the voltages induced at the first and second windings, respectively.
In another embodiment of the present invention, the capacitor is connected in series with the secondary winding and the rectifier across the DC power source in parallel with a series combination of the primary winding and the switching element. Thus, there is established a closed loop of the capacitor, the secondary winding, the rectifier and the converter input for flowing the input current therethrough while the switching element is off. To this end, the secondary winding has a polarity chosen in relation to the winding sense of the primary winding such that the input DC voltage is superimposed in phase upon the voltage induced at the secondary winding. Alternatively, the secondary winding may have a polarity chosen in relation to the winding sense of the primary winding such that the input DC voltage is superimposed in reverse phase upon the voltage induced at the secondary winding.
Preferably, a low-pass filter is connected across the converter output in order to remove output ripples.
The DC-DC converter can be best applied to a ballast for a discharge lamp in which an inverter is connected to convert the output DC voltage from the converter into an AC voltage for operating the discharge lamp.